The present invention relates to a control system for controlling the distribution of the driving force between front and rear wheel pairs of a four wheel drive vehicle by varying the driving force transmitted to either of the front and rear wheel pairs.
One conventional example of the four wheel drive system is shown in FIG. 9 (cf. "Jid o sha Ko gaku" vol. 33, no. 8, published by Tetsudo Nihon Company Ltd. Aug. 1, 1984, pages 56-62). This system is a so-called full-time four wheel drive system having a center differential between a front wheel propeller shaft and a rear wheel propeller shaft, and a dog clutch for locking the center differential. In this four wheel drive system, the driving force distribution ratio between the front wheel driving force and the rear wheel driving force is 1:1. Therefore, when the center differential is locked by the dog clutch, this system increases the tendency to understeer although it can improve the limit cornering performance of the vehicle. When the center differential is unlocked, this system can solve the problem of understeer, but it deteriorates the limit cornering performance.
Furthermore, it is impossible to control the direction of the vehicle with the aid of power slide obtained by operation of the accelerator pedal.
In another conventional example, a drive system is switched between a two wheel drive mode and a four wheel drive mode by a clutch in a simple on-off manner. In this system, the changeover between the 2WD and 4WD modes during motion of the vehicle is difficult, and even dagerous because an abrupt change of the drive state impairs the vehicle stability.